Introduce CI workflow running cargo audit

In order to continuously monitor our dependencies for security
vulnerabilities, we introduce a new CI job that will use `cargo audit`
to check for any known vulnerabilities.

This job is run on a daily schedule, or whenever a `Cargo.toml` file is
changed. For each new advisory, a new issue will be created.

Author: tnull

.github/workflows/audit.yml

@@ -0,0 +1,16 @@
+name: Security audit
+on:
+  schedule:
+    - cron: '0 0 * * *'
+  push:
+    paths:
+      - '**/Cargo.toml'
+      - '**/Cargo.lock'
+jobs:
+  security_audit:
+    runs-on: [ubuntu-latest, macos-latest]
+    steps:
+      - uses: actions/checkout@v3
+      - uses: rustsec/audit-check@v1.4.1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}